old external hard drive with new mac

I suppose HDD is dead, though Disk Utility detects that external drive when you connect it to iMac?

There is a chance that a hardware problem in the external enclosure is preventing

the hard drive within it from being read or seen correctly by the computer system.

A 'bare drive wire kit' such as one shown at owc macsales.com could be used to

test and see if the drive once removed from the enclosure, could be accessed

and any desired data retrieved. The kit has wire connections and power adapter.

NewerTech Universal Drive Adapter USB 3.0 - connects any 2.5", 3.5" or 5.25" drive

Something similar may be available from a local computer shop, be sure to check

on their return policy if the unit does not work out for you. The above linked item is

among the better ones available and is from a reputable vendor of Mac-centric stuff.

Could be time to consider a new enclosure with faster connection ports for a wider

usefulness & later purposes such as backups, with sufficient capacity for partitions.

I'd not put an old HDD into a new enclosure, but the universal wire kit would solve

the question about accessing some old drive locked in potentially failed enclosure.

In any event...

Good luck & happy computing! 🙂

The internal circuitry that supports power and data to and from the externally enclosed

hard disk drive may likely have a failure in one or another connection or hardware part.

So to remove the hard drive from the enclosure and attach it more directly in attempt

to retrieve the content is a suggested method to try & recover information, since there

is a fair chance the hard drive itself is not the main problem. It may be OK enough to

use a universal USB cable kit to at least test the theory, since it isn't terribly expensive.

Sometimes the format used in an old external drive may not be suitable for Mac OS X

but may be OK for some kinds of files or to be shared with a Windows PC; however

the sound of your issue appears to point to a failure to communicate between device

and computer. A bad cable or printed circuit inside the enclosure, a partially failed

power supply inside the enclosure, & so on. And the old hard drive may be worn out.

Hopefully you can at least recover the content of the old hard drive; be sure that any

computer you use to recover to, has a known-good additional external HDD attached

so the data won't have to be saved into the computer directly. Especially if the files

may over-fill the computer's internal hard drive.

Good luck in this matter! 🙂